Cork devices have for many years been attached to items such as keys to enable them to float and be retrievable if dropped into water, for example in harbours and marinas. These cork devices are relatively bulky and are not sufficiently buoyant to support heavy items.
More recently, various proposals have been made, for example in FR 2733482, US 2004/0137810 and GB 2406265 A, for devices for this purpose which incorporate an automatically inflatable bag to serve as the float. These devices are all impractical to produce economically, or else are unsafe or unreliable in operation. For example, FR 2733482 proposes a cylinder in which acetylene gas is generated by a chemical reaction upon contact of calcium carbide with water. Even if this reaction was reliably achieved, it is clearly unsafe for use as acetylene is flammable. The proposal in US 2004/0137810 involves breakdown in water of a barrier between two compartments within the inflatable balloon, which would be most unreliable in practice. The proposal in GB 2406265 A and other known devices involve use of a canister of compressed carbon dioxide and a trigger device which involves piercing of a seal to this canister, the trigger device being dependent on breakdown of a material on contact with water. These are expensive to produce because a canister of compressed carbon dioxide must be of metal and sufficiently robust to withstand the high pressure required to contain the gas. Moreover, they are unreliable in use in part because of frequent occurrence of air locks in the passage of water to the water degradable material, but also because the force required to pierce such a seal is considerable and may not be achieved.